Current differential protection is a reliable and widely used method for protection in electrical power systems. It is based on the idea of measuring currents on both sides of a protected zone or both sides of a protected object and calculating the difference between them, giving a differential current. The currents are typically measured by means of current transformers (CTs). A protected object or zone may be any part of the power system, for example a transmission line, transformer, generator, or a busbar.
FIG. 1 is a graph showing typical operate-restrained characteristics of the current differential protection. The differential current IDIFF (y-axis) may differ from (expected) zero even when there is no internal fault, e.g. due to current transformer errors. In common for most implementations is that an operate signal (trip command) is given when the differential signal is above a set pick-up value, which is a differential characteristic pick-up setting that accounts for errors causing false differential currents. That is, the set pick-up value is used for securing against false differential currents, thereby not restraining the protected object unnecessarily.
The differential current ID (y-axis), also denoted operating current, operation level, or operating point, is a function of a bias current IBIAS (x-axis) (also denoted restraining current). The restraint characteristic is a measure on amount of current that a protection relay, that implements the current differential protection, will use to restrain the protected object. The amount of current is based on the currents measured at the respective CT locations. Basically, as long as the differential current is lower than the differential characteristic pick-up value (i.e. lies in the area denoted Restraint region in FIG. 1) the power system is operational, and if the differential current is higher than the differential characteristic pick-up value (i.e. lies in the area denoted Operate region in FIG. 1) then circuit breakers should be tripped (opened) for avoiding damaging the object protected by the current differential protection.
The operate-restrain characteristic is intended to secure the operation of the current differential protection during external faults that have very high fault currents. During such external faults, the extremely high fault currents may bring large errors on the measured current values and might also saturate a primary CT, which in turn may cause mal-operation of the differential protection. In order to prevent mal-operation due to the CT saturation, additional logics can be applied to block the differential protection function when the CT saturates during the external faults. Harmonic blocking, internal/external fault discriminator based on negative/zero sequence currents are examples of such function. However, these functions (or methods) cannot always prevent the current differential protection mal-operation.